In telephone engineering, it is usual practice to provide protectors at central offices for each incoming line. These protectors, which may be termed modules, combine protection against excessive voltages resulting from lightning, for example, with protection against sneak currents. Sneak currents are not strong enough to do any damage if they flow briefly, but may generate enough heat to char conductor insulation and do other damage if allowed to persist. The sneak currents are produced by voltages of relatively low magnitude as compared to the excessive voltages mentioned hereinabove and usually result from accidental interference between telephone lines and adjacent power lines.
Protection of a telephone line against excessive voltage is usually provided by a so called spark-gap protector which generally includes a pair of spaced carbon electrodes or electrodes of a gaseous discharge tube. One of the electrodes is usually connected to ground and the other to the incoming telephone line. Should a high voltage be impressed on the line, it will bridge the gap between the electrodes and cause current to flow to ground, thus bypassing sensitive equipment which is associated with the line.
The second type of protection is commonly provided by a device that is referred to as a heat coil. The heat coil includes insulated high resistance wire which is wound on a metal sleeve inside of which a contact pin is held in a predetermined position by a fusible bonding material such as solder, for example. Should excessive currents occur on the line and persist, sufficient heat will be generated by the wire to melt the solder and release the pin. A spring is usually provided to urge the released pin into electrical contact with a source of ground potential to ground the line.
Inasmuch as a ring conductor and a tip conductor are associated with each telephone station apparatus, each telephone line requires two protector assemblies. A telephone circuit protector module shown in J. B. Geyer et al U.S. Pat. No. 3,573,695 which issued on Apr. 6, 1971, includes two protector assemblies enclosed in a single insulative housing. Spark-gap and heat coil subassemblies therein are held in abutting aligned relation by a single spring which is part of the normal transmission circuit. The spring also serves to propel a pin of the heat coil subassembly into engagement with a grounding circuit, which includes one of two carbon blocks, during the passage of excessive currents through the heat coil. The extension of a contact pin through voltage protection portions of the protector precludes the use of gaseous discharge devices in place of carbon blocks. Gaseous discharge devices, which are commonly referred to as gas tubes, are desirable because of their longer lives and because they afford better control of the breakdown voltage.
In a protector module shown in U.S. Pat. No. 4,215,381 which issued on July 29, 1980 to R. F. Heisinger, gaseous discharge devices may be used inasmuch as the voltage protection portion of the protector is taken out of the fault circuit. When sufficient heat is transferred to the heat coil subassembly such as by a current fault, a fusible alloy melts to allow a spring to cause a heat coil flange to move and touch a laterally projecting tab of a ground terminal assembly. If a prolonged voltage surge occurs, there is an arcing over in the voltage surge limiter assembly, heat energy is transferred to a pin of the heat coil which engages a portion of the voltage surge limiter assembly, the fusible alloy is melted, and the spring moves the heat coil flange plate as before. However, the Heisinger protector module continues the use of a spring as part of the normal transmission and fault current circuits. At times, the presence of the spring in the voice frequency circuit may result in noise on the line. Also, because the spring moves slidably, insulating sleeves are disposed about the spring to prevent shorting.
A protector assembly having substantially lower elements and adapted to include either gas tubes or carbon blocks is disclosed in U.S. Pat. No. 4,458,288 which issued on July 3, 1984 in the names of J. L. Chapman, Jr. et. al. Each of two protector assemblies supported in a common housing includes a current protection subassembly which comprises a dielectric base and a line pin and a central office pin connected together electrically. A shunting element is disposed concentrically about the line pin and is secured to one end of the line pin in an initial position by a fusible material. A spring which is located between a cup of each voltage protection subassembly and the housing and which is not in the transmission circuit maintains the voltage protection subassembly in engagement with the shunting element. The spring is effective when current flow exceeds a predetermined level that is sufficient to melt the fusible material to cause the shunting element to be moved to a position where it engages a portion of a grounding subassembly to establish a fault current path to ground. For a prolonged voltage surge, heat energy is transferred from the voltage protection subassembly to the shunting element and melts the fusible material to allow the shunting element to be moved as in a current overload mode.
In a gas tube type protector, the electrodes typically are supported in an insulative sleeve housed in a metallic cup. The metallic cup is connected to ground. An end of one of the electrodes protrudes from the cup into engagement with the flanged portion of the sleeve positioned on the line pin. The level of the breakdown voltage is controlled by disposing a gas between the electrodes or by controlling the pressure of the air therebetween or by a combination of these measures.
In a narrow gap gas tube, the gap between electrodes is about 0.002 inch. Should the gas in the tube vent and there be an excessive voltage, the voltage will arc across the narrow gap and be conducted to ground. Such an arrangement offers protection in the 600 to 1200 volt range; however, a narrow gap gas tube is somewhat costly to manufacture.
Instead of a narrow gap gas tube, one with a wide gap, which is less costly to manufacture, may be used. In such a device, the gap is in the range of about 0.017 to 0.040 inch. If the gas vents, arcing across the wide gap will occur at a higher voltage, compromising the protection. It would be most desirable to have a fail-safe device with a high level of confidence for protection in the event the gas vents. Such a fail-safe device should be one which is incorporated easily into the above-described protector and which cooperates with other elements thereof to provide a path to ground in the event of an excessively high voltage.
Such fail-safe devices are not new in the art. For example, in one prior art arrangement, a washer which is made of a dielectric material is disposed externally of a gas tube to provide a secondary or auxiliary gap. However, the manner in which it is used and its cooperation with other elements seemingly does not lend itself well to manufacture. Also, in at least one patent which discloses a surge voltage arrester assembly, a first conductive member which engages a first electrode of the arrester is held in engagement with a metallic contact member. A solder pellet is supported between a cage and the second electrode of the surge arrester and a spring is interposed between the cage and a tubular housing member. Auxiliary gap protection is provided between the first conductive member and a second conductive member by an insulator disc having openings therein. The second conductive member is held in engagement with the insulator disc by fingers of the cage. In these kinds of arrangements, melting of the solder pellet allows the spring to move the cage downwardly causing disengagement of the fingers and the second conductive member. As a result, integrity of the auxiliary gap may be compromised. What is needed and what seemingly is not provided in the prior art is a wide gap gas tube protector having a secondary or auxiliary gap for excessively high voltage and being arranged so that the integrity of the auxiliary gap is maintained.